1. Field of the Invention
The present invention relates to an organic electroluminescent display device.
2. Description of the Related Art
Display devices using organic electroluminescent (EL) elements have gotten much attention recently as display devices which can be made thinner and lighter than conventional CRT and LCD display devices. Being self-luminous, the organic electroluminescent elements have various advantages such as high level of visibility, no dependence on viewing angles, use of flexible film substrates, and their slimness and light weight in comparison with liquid crystal display devices.
Further, in order to reduce response time and realize high-definition display, an active matrix display device in which thin film transistors are used as switching elements for driving organic electroluminescent elements has been proposed.
FIG. 16 is a cross-sectional view showing the structure of a proposed active matrix organic electroluminescent display device. Note that FIG. 16 shows the structure of a portion of the display device corresponding to one pixel, but in practice plural pixels are arranged in the form of a matrix.
As shown in the figure, a buffer layer 102 is formed on an insulating substrate 100 formed of glass. A channel layer 104 is formed on the buffer layer 102. A gate electrode 108 is formed on the channel layer 104 via a gate insulating layer 106. A source region 110 and a drain region 112 are formed at the channel layer 104 at the sides of the gate electrode 108. In this way, a thin film transistor having the gate electrode 108, the source region 110 and the drain region 112 is formed on the buffer layer 102.
An interlayer insulating layer 114 is formed on the buffer layer 102 having the thin film transistor formed thereon. A source electrode 118 connected via a contact hole 116 to the source region 110 and a drain electrode 122 connected via a contact hole 120 to the drain region 112 are formed on the interlayer insulating layer 114.
An interlayer insulating layer 124 is formed on the interlayer insulating layer 114 on which the source electrode 118 and the drain electrode 122 are formed. A contact hole 126 which reaches the source electrode 11 8 is formed in the interlayer insulating layer 124.
On the interlayer insulating layer 124 having the contact hole 126 formed therein, an organic electroluminescent element having a lower electrode 128 formed by a transparent conductive layer such as an ITO (indium tin oxide) layer, an organic electroluminescent layer 130, and an upper electrode 132 formed by an Al (aluminum) layer or a Mg—Ag alloy layer is formed at a portion including the contact hole 126. The organic electroluminescent layer 130 is formed by sequentially laminating, for example, a hole transport layer, a luminescent layer and an electron transport layer. The lower electrode 128 is electrically connected to the source electrode 118 via the contact hole 126 formed in the interlayer insulating layer 124.
In the proposed organic electroluminescent display device shown in FIG. 16, a layer which does not transmit light such as the Al layer is used as the upper electrode 132. Thus, in the organic electroluminescent display device shown in FIG. 16, light generated at the luminescent layer of the organic electroluminescent layer 130 exits from the side of the insulating substrate 100. The organic electroluminescent display device having such a structure is called a bottom emission organic electroluminescent display device.
In order to realize full-color display at the organic electroluminescent display device shown in FIG. 16, luminescent layers having different emission wavelengths need to be formed at a pixel area. For example, a mask having an opening at a portion corresponding to the pixel area is made to closely contact the substrate. Luminescent layers forming the respective colors of R, G and B are formed by moving the masks in the order of, for example, R, G and B.
In the case of the bottom emission organic electroluminescent display device, as described above, light generated at the organic electroluminescent element exits from the insulating substrate side. In the organic electroluminescent display device shown in FIG. 16, since the switching element is formed between the insulating substrate and the organic electroluminescent element, an emission area for a pixel is practically decreased due to the existence of the switching element. As a result, high brightness cannot be achieved.
In order to improve brightness, it has also been proposed to make light, which is generated at the luminescent layer, exit from the side of the upper electrode. The organic electroluminescent display device having such a structure is called a top emission organic electroluminescent display device.
FIG. 17 is a cross-sectional view showing the structure of a proposed top emission organic electroluminescent display device.
As shown in the figure, an upper electrode 134 which transmits light and is formed by a transparent conductive layer such as an ITO layer is used as the upper electrode of the organic electroluminescent element. With this structure, light generated at the luminescent layer of the organic electroluminescent layer 130 exits from the side of the upper electrode 134. In the top emission organic electroluminescent display device, light generated at the organic electroluminescent element is not prevented by the thin film transistor from exiting. Thus, according to the proposed organic electroluminescent display device shown in FIG. 17, the aperture ratio of a pixel can be improved. In FIG. 17, the reference characters other than the above have the same meanings as in FIG. 16.
However, the upper electrode for the top emission organic electroluminescent display device needs to be thin in order to ensure light transmission. Further, the transparent conductive layer such as the ITO layer used as the upper electrode has a resistance larger than that of a metal layer or the like. For this reason, the resistance of the upper electrode may increase to cause voltage drop, thereby resulting in deterioration of display performance such as degradation of brightness or uneven brightness.
As a countermeasure for suppressing voltage drop at the upper electrode, it has been proposed to connect an auxiliary wiring formed of metal to the upper electrode (see, for example, Japanese Patent Application Laid-Open Nos. 2001-195008, 2004-207217, 2002-318556 and 2002-352963).
For example, Japanese Patent Application Laid-Open No. 2001-195008 discloses an organic electroluminescent display device in which a rib serving as a spacer for a mask and also as an auxiliary wiring for a common upper electrode is provided for each pixel.
Further, Japanese Patent Application Laid-Open No. 2004-207217 discloses an organic electroluminescent display device in which an upper electrode is connected via a connection hole to an auxiliary wiring, which is formed in the same layer as a lower electrode and disposed such that the auxiliary wiring is insulated from the lower electrode.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2001-195008, however, the size of a pixel is decreased by the rib serving as the auxiliary wiring. Thus, it is difficult to achieve sufficiently high brightness.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2004-207217 as well, the size of a pixel is decreased by the auxiliary wiring. Thus, it is difficult to achieve sufficiently high brightness.